Oxygen vacancy ordering in CaTiO3-CaFeO2.5 perovskites: From isolated defects to infinite sheets

The oxygen vacancy ordering process in the system CaTiO3-CaFeO2.5 has been studied as a function of composition and temperature. The samples (CaTi1-xF exO3-x/2 with 0 less than or equal to x less than or equal to 1) were annea led at temperatures between 900 degrees C and 1400 degrees C and characteri zed, after drop quench, by means of XRD, Mossbauer spectroscopy and TEM. Va cancy ordering starts from isolated oxygen vacancies randomly distributed i n the lattice. They first cluster into chains of finite length which increa ses with increasing Fe content or decreasing temperature. Eventually, the c hains become infinitely long and arrange in planes with tetrahedrally coord inated cations that alternate with layers of octahedrally coordinated ones. Long-range order of T (tetrahedral) and O (octahedral) planes develops in samples with x greater than or equal to 0.50. Mossbauer area ratio data ind icate that at low Fe contents Ti must be present, partially, in pentacoordi nated sites while at higher x this cation must be distributed over octahedr al as well as tetrahedral sites.